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Moon, Saturn, and Spica

StarDate:

February 2, 2010

If you think that all those little points of light in the night sky look the same, it probably means you're not really looking at them. If you watch them carefully, you'll see differences in brightness, color, and even how steadily they shine.

A couple of examples flank the Moon late tonight: the star Spica and the planet Saturn. They rise by around midnight, and are in the southwest at first light tomorrow. Saturn is to the upper left of the Moon, with Spica to the lower left.

At first glance, these two pinpoints of light look about the same. But with a little longer glance, you should see some differences.

Saturn, for example, is a little brighter than Spica. And while Saturn looks golden, Spica is white with a hint of blue. Spica's color comes from its surface temperature -- it's thousands of degrees hotter than the Sun. Saturn, on the other hand, shines by reflecting sunlight. Its color comes from ammonia and other chemicals in its upper atmosphere.

And while Spica twinkles, Saturn remains pretty steady. That's because Spica is so far away that it really is only a pinpoint of light. That light is bent and split apart as it passes through different layers of the atmosphere. But Saturn is close enough that it forms a tiny disk in the sky. Each point of light from Saturn is also distorted by the atmosphere, but the points all blur together to keep the planet shining steadily.

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